Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 AM EST Mon Feb 05 2024

Valid 12Z Mon Feb 05 2024 - 12Z Thu Feb 08 2024

...The West...
Days 1-3...

An amplifying trough just off the CA coast into Wednesday will
continue to direct an impressive moisture plume onshore and spread
it into much of the West as an atmospheric river (AR) persists
early this week. The greatest IVT probabilities within this IVT
are progged to make landfall across central/southern CA overnight
tonight (so before the D1 period) thanks to embedded shortwaves
within confluent mid-level flow south of a potent, but weakening,
shortwave advecting near the CA/OR border. Even as this feature
and the IVT weakens, persistent W/SW onshore flow will remain
confluent into the CA coast, suggesting persisting IVT into
Tuesday. Thereafter, a secondary surge of PW/IVT is progged to
develop as the trough along the CA coast re-amplifies into a
closed low and then advects onshore late Tuesday. In response to
this secondary amplification, both the GEFS and ECENS suggest high
probabilities for 250 kg/ms IVT surging from Baja California, with
actually the greatest chance for IVT exceeding 250kg/ms across the
Intermountain West occurring during this time. This is further
reflected by NAEFS PW anomalies of more than +1 sigma expanding
across much of the West by Wednesday.

In this increasingly saturated environment, forcing will remain
broad but pronounced within large scale cyclonic flow that will
envelop the West. Spokes of vorticity associated with weak
impulses moving within the flow will help drive locally enhanced
ascent at times, combining with a potent and persistent SW to NE
oriented jet streak and regions of more impressive upslope flow
where terrain is orthogonal to the mean wind to produce areas of
heavy snow through the period. The heaviest precipitation which
will drive this heavy snow will likely shunt E/SE with time
through Wednesday, and although snow levels during the periods of
heaviest precipitation will generally be 6000-7000 ft, they should
fall by the end of the period to around 4000 ft, supporting some
lower elevation accumulations by D3.

On D1, WPC probabilities or more than 6 inches are highest in the
Shasta/Trinity/Siskiyou region, as well as along the length of the
Sierra where they exceed 90%, and snowfall of 1-3 feet is likely
above 6000 ft. Additional heavy snow of more than 1 foot is likely
in other favored upslope ranges including the Wind Rivers, Tetons,
San Bernadinos, Spring Mountains, and other ranges of the Great
Basin. During D2, the heaviest precipitation begins to shift east,
focusing heavy snow from the southern Sierra and Transverse Ranges
of Southern CA, eastward through the Four Corners including the
Mogollon Rim, White Mountains of AZ, San Juans, and along the
Wasatch, as well as continuing across much of the higher terrain
of NV. The continued SW flow in anomalous moisture within broad
deep layer ascent will result in another day of heavy snow across
many of these same ranges D3, with some extension northward
towards the Absarokas. 3-day snowfall in many of these ranges,
especially above 6000 ft, could exceed 3 feet, and considerable
impacts to travel are possible at many of the mountain passes.


...Northern High Plains...
Day 3...

A shortwave ejecting from the Four Corners Wednesday morning will
sharpen and amplify into a closed low over the Northern Plains
Wednesday night as it advects rapidly to the northeast. This will
occur on the eastern edge of a broad trough encompassing the
Intermountain West, resulting in upper level jet streak
intensification downstream of the primary trough axis. This jet
streak will arc poleward while strengthening to 130+ kts, placing
its favorable LFQ for diffluence into the Northern Plains. This
will overlap effectively the closed mid-level low, supporting a
deepening surface low pressure from eastern CO through ND by the
end of the forecast period.

This ascent will act upon an increasingly moistening column as
deep 700-500mb SW flow from the Pacific merges with increasing
low-level flow out of the Gulf of Mexico downstream of the
strengthening surface low. The resultant theta-e ridge will surge
into the Dakotas by Wednesday evening, and may pivot cyclonically
into a TROWAL late, to additionally enhance lift across the
region. With the column expected to be sufficiently cold west of
the surface low, the primary 290-295K isentropic ascent will drive
an expansion of snowfall across the Northern High Plains. There is
still considerable spread in the low track so confidence is modest
into placement of the heaviest snow axis, but the overlap of
forcing and moisture should be sufficient for a swath of heavy
snow, especially late D3, and current WPC probabilities for more
than 4 of snow are as high as 30% in northeast MT.


The probability of significant icing greater than 0.10" is less
than 10% for days 1-3.


Key Messages for the Feb 4-7 Atmospheric River are below:

--Life Threatening Flooding and Rainfall
Heavy rain will continue today for much of California, with rain
persisting in southern California on Tuesday. Additional rainfall
of 1-3" is forecast, with locally 5-8" possible in parts of
Southern California and the Transverse Ranges. Locally
catastrophic flash and urban flooding impacts are likely to
continue across Los Angeles. Small stream and river flooding with
debris flows and mudslides are also expected.

--Heavy Mountain Snow and Whiteouts
Heavy, wet snow will continue across the Sierra Nevada and near
Mt. Shasta through tonight. Snowfall rates at times will reach
2-3"/hr, accumulating to multiple feet. These rates combined with
wind gusts exceeding 60 mph will produce whiteout conditions and
near impossible travel above 5000 feet. Southern California ranges
can expect heavy snow through Tuesday above 7000 feet causing
similar impacts.

--Damaging Winds and High Surf
Powerful, damaging onshore winds will continue across central and
southern California through late this morning before slowly
waning. Further power outages and downed trees are likely.
Potentially damaging high surf is expected for the Central and
Southern California coasts through Monday.


Weiss